Histone deacetylase (HDAC) proteins play a critical role in regulating gene expression in vivo by altering the accessibility of genomic DNA to transcription factors. Specifically, HDAC proteins remove the acetyl group of acetyl-lysine residues on histones, which can result in nucleosomal remodeling (Grunstein, M., 1997, Nature, 389: 349-352). Due to their governing role in gene expression, HDAC proteins are associated with a variety of cellular events, including cell cycle regulation, cell proliferation, differentiation, reprogramming of gene expression, and cancer development (Ruijter, A-J-M., 2003, Biochem. J., 370: 737-749; Grignani, F., 1998, Nature, 391: 815-818; Lin, R-J., 1998, 391: 811-814; Marks, P-A., 2001, Nature Reviews Cancer, 1: 194). The aberrant deacetylation resulting from the misregulation of histone deacetylases (HDACs) has been linked to clinical disorders such as Rubinstein-Taybi syndrome, fragile X syndrome, leukemia, and various cancers (Langley B et al., 2005, Current Drug Targets—CNS & Neurological Disorders, 4: 41-50). In fact, HDAC inhibitors have been demonstrated to reduce tumor growth in various human tissues and in animal studies, including lung, stomach, breast, and prostrate (Dokmanovic, M., 2005, J. Cell Biochem., 96: 293-304).
The aberrant histone deacetylase activity has also been linked to various neurological and neurodegenerative disorders, including stroke, Huntington's disease, Amyotrophic Lateral Sclerosis and Alzheimer's disease. HDAC inhibition may induce the expression of anti-mitotic and anti-apoptotic genes, such as p21 and HSP-70, which facilitate survival. HDAC inhibitors can act on other neural cell types in the central nervous system, such as reactive astrocytes and microglia, to reduce inflammation and secondary damage during neuronal injury or disease. HDAC inhibition is a promising therapeutic approach for the treatment of a range of central nervous system disorders (Langley B et al., 2005, Current Drug Targets—CNS & Neurological Disorders, 4: 41-50).
Mammalian HDACs can be divided into three classes according to sequence homology. Class I consists of the yeast Rpd3-like proteins (HDAC 1, 2, 3, 8 and 11). Class II consists of the yeast HDA1-like proteins (HDAC 4, 5, 6, 7, 9 and 10). Class III consists of the yeast SIR2-like proteins (SIRT 1, 2, 3, 4, 5, 6 and 7).
The activity of HDAC1 has been linked to cell proliferation, a hallmark of cancer. Particularly, mammalian cells with knock down of HDAC1 expression using siRNA were antiproliferative (Glaser, K-B., 2003, Biochem. Biophys. Res. Comm., 310: 529-536). While the knock out mouse of HDAC1 was embryonic lethal, the resulting stem cells displayed altered cell growth (Lagger, G., 2002, EMBO J., 21: 2672-2681). Mouse cells overexpressing HDAC1 demonstrated a lengthening of G2 and M phases and reduced growth rate (Bartl. S., 1997, Mol. Cell. Biol., 17: 5033-5043). Therefore, the reported data implicate HDAC1 in cell cycle regulation and cell proliferation.
HDAC2 regulates expression of many fetal cardiac isoforms. HDAC2 deficiency or chemical inhibition of histone deacetylase prevented the re-expression of fetal genes and attenuated cardiac hypertrophy in hearts exposed to hypertrophic stimuli. Resistance to hypertrophy was associated with increased expression of the gene encoding inositol polyphosphate-5-phosphatase f (Inpp5f) resulting in constitutive activation of glycogen synthase kinase 3β (Gsk3β) via inactivation of thymoma viral proto-oncogene (Akt) and 3-phosphoinositide-dependent protein kinase-1 (Pdk1). In contrast, HDAC2 transgenic mice had augmented hypertrophy associated with inactivated Gsk3β. Chemical inhibition of activated Gsk3β allowed HDAC2-deficient adults to become sensitive to hypertrophic stimulation. These results suggest that HDAC2 is an important molecular target of HDAC inhibitors in the heart and that HDAC2 and Gsk3β are components of a regulatory pathway providing an attractive therapeutic target for the treatment of cardiac hypertrophy and heart failure (Trivedi, C-M., 2007, Nat. Med. 13: 324-331).
HDAC3 are maximally expressed in proliferating crypt cells in normal intestine. Silencing of HDAC3 expression in colon cancer cell lines resulted in growth inhibition, a decrease in cell survival, and increased apoptosis. Similar effects were observed for HDAC2 and, to a lesser extent, for HDAC1. HDAC3 gene silencing also selectively induced expression of alkaline phosphatase, a marker of colon cell maturation. Concurrent with its effect on cell growth, overexpression of HDAC3 inhibited basal and butyrate-induced p21 transcription in a Sp1/Sp3-dependent manner, whereas silencing of HDAC3 stimulated p21 promoter activity and expression. These findings identify HDAC3 as a gene deregulated in human colon cancer and as a novel regulator of colon cell maturation and p21 expression (Wilson, A-J., 2006, J. Biol. Chem., 281: 13548-13558).
HDAC6 is a subtype of the HDAC family that deacetylates alpha-tubulin and increases cell motility. Using quantitative real-time reverse transcription polymerase chain reaction and Western blots on nine oral squamous cell carcinoma (OSCC)-derived cell lines and normal oral keratinocytes (NOKs), HDAC6 mRNA and protein expression were commonly up-regulated in all cell lines compared with the NOKs. Immunofluorescence analysis detected HDAC6 protein in the cytoplasm of OSCC cell lines. Similar to OSCC cell lines, high frequencies of HDAC6 up-regulation were evident in both mRNA (74%) and protein (51%) levels of primary human OSCC tumors. Among the clinical variables analyzed, the clinical tumor stage was found to be associated with the HDAC6 expression states. The analysis indicated a significant difference in the HDAC6 expression level between the early stage (stage I and II) and advanced-stage (stage III and IV) tumors (P=0.014). These results suggest that HDAC6 expression may be correlated with tumor aggressiveness and offer clues to the planning of new treatments (Sakuma, T., 2006, Int. J. Oncol., 29: 117-124).
Epigenetic silencing of functional chromosomes by HDAC is one of the major mechanisms that occurs in pathological processes in which functionally critical genes are repressed or reprogrammed by HDAC activities, leading to the loss of phenotypes in terminal differentiation, maturation and growth control, and the loss of functionality of tissues. For example, tumor suppressor genes are often silenced during development of cancer and chemical inhibitors of HDAC can derepress the expression of these tumor suppressor genes, leading to growth arrest and differentiation (Glaros S et al., 2007, Oncogene June 4 Epub ahead of print; Mai, A, et al., 2007, Int J. Biochem Cell Bio., April 4, Epub ahead of print; Vincent A. et al., 2007, Oncogene, April 30, Epub ahead of print; our unpublished results). Repression of structural genes such as FXN in Friedreich's ataxia and SMN in spinal muscular atrophy can be reversed by HDAC inhibitors, leading to re-expression and resumption of FXN and SMN gene function in tissues (Herman D et al., 2006, Nature Chemical Biology, 2(10):551-8; Avila A M et al., 2007, J Clinic Investigation, 117(3)659-71; de Bore J, 2006, Tissue Eng. 12(10):2927-37); Induction of the entire MHC II family gene expression through reprogramming of HDAC “hot spot” in chromosome 6p21-22 by HDAC inhibitors further extends epigenetic modulation of immune recognition and immune response (Gialitakis M et al., 2007, Nucleic Acids Res., 34(1); 765-72).
Several classes of HDAC inhibitors have been identified, including (1) short-chain fatty acids, e.g. butyrate and phenylbutyrate; (2) organic hydroxamic acids, e.g. suberoylanilide hydroxamic acid (SAHA) and trichostatin A (TSA); (3) cyclic tetrapeptides containing a 2-amino-8-oxo 9,10-expoxydecanoyl (AOE) moiety, e.g. trapoxin and HC-toxin; (4) cyclic peptides without the AOE moiety, e.g. apicidin and FK228; and (5) benzamides, e.g. MS-275 (EP0847992A1, US2002/0103192A1, WO02/26696A1, WO01/70675A2, WO01/18171A2). HDAC represents a very promising drug target especially in the context of epigenic biology; for example, in terms of preferential apoptosis-induction in malignant cells but not normal cells, differentiation of epithelia in cancer cells, anti-inflammatory and immunomodulation, and cell cycle arrest.
The use of HDAC inhibitors can be considered as “neo-chemotherapy” having a much improved toxicity profile over existing chemotherapy options. The success of SAHA from Merck is currently only limited to the treatment of cutaneous T cell lymphoma. No reports exist indicating that SAHA treatment is effective against major solid tumors or for any other indications. Therefore, there is still a need to discover new compounds with improved profiles, such as stronger HDAC inhibitory activity and anti-cancer activity, more selective inhibition on different HDAC subtypes, and lower toxicity; There is also a continuing need to identify novel HDAC inhibitors that can be used to treat potential new indications such as neurological and neurodegenerative disorders, cardiovascular disease, metabolic disease, and inflammatory and immunological diseases.